1. Field of the Invention
This invention relates to methods of joining components.
More particularly the invention relates to methods of joining components by soldering, suitable for use in joining a silicon wafer to a component of molybdenum or tungsten.
2. Description of Related Art
Such methods find application in the manufacture of high power silicon semiconductor devices. In such devices the electrical connections to the silicon wafer in which the device is formed are conventionally made by way of relatively massive copper electrodes, one on each side, which also serve to conduct heat away from the wafer. Ideally the wafer would be bonded directly to these copper electrodes. In practice the difference between the thermal expansion coefficients of copper and silicon renders this impracticable. Instead, one main face of the wafer is conventionally brazed to a molybdenum or tungsten backing plate, normally in the form of a disc, and a dry sliding contact between one copper electrode and the disc is made. The other copper electrode makes a dry sliding contact with another molybdenum disc which in turn makes a dry sliding contact with the main face of the water remote from the brazed disc, the wafer being unable to withstand the stress associated with two brazed joints.
In conventional high-power silicon semiconductor device manufacturing processes the wafer is brazed to the disc using an Al-12%wtSi braze the melting point of which is 577.degree. C. The use of a braze of such a high melting point introduces stresses in the brazed assembly which give rise to bowing. This can be largely removed by clamping under pressure in the final encapsulated package, but the stresses remain and these can impair performance of the device.